In multi-access communication systems, or networks, for interconnecting multiple devices or stations in a communications network, transceivers are provided for controlling access of the devices to the communications channel. In one type network, a single coaxial cable interconnects devices for communications with each other. Of course, only one device can transmit at any given time over such a cable, if all devices operate at the same frequency. One form of control which has evolved for such networks is referred to as carrier-sense multiple access with collision detection ("CSMA/CD"). With this technique, each device controls its own access to the coaxial cable channel. Each device which uses the channel interfaces to the cable through a transceiver which includes apparatus for transmitting a signal onto the channel as well as apparatus for receiving a signal from the channel placed thereon by another device's transceiver. The transceivers each include a collision detector for generating a collision signal whenever a signal transmitted on the cable by another transceiver is detected at the same time the transceiver itself is transmitting onto the channel. Each transceiver sends the collision signal back to its host device and in response both hosts stop transmitting. Both then retry transmission after the channel is clear.
Each device that wants to use (i.e., transmit on) the channel, first "listens" to the receiver in its transceiver to hear if any other station is transmitting. If it detects no other host transmitting, the station starts its transmission, while receiving at the same time. Another station also might start transmitting, though. If that happens, both detect the collision and stop, as noted above. To avoid repeated collisions, each then waits a pseudorandom interval and tries again. On each retry, the pseudorandom delay is increased.
Such a system is illustrated, for example, in U.S. Pat. No. 4,063,220, issued Dec. 13, 1977 to Robert M. Metcalfe et al.
In such CSMA/CD networks, the transceivers must be able to determine whether the network is being used by no transmitters, one transmitter or two (or more) transmitters. This requires comparison of the signal on the cable with precise threshold levels. However, the voltage supplied by the host device to operate its associate transceiver ordinarily is not very accurately established at a known level. Consequently, a precisely adjustable, regulated power supply is needed to establish the threshold voltages used by the transceiver.
The various objects, features and advantages of the present invention will be understood from the following description.